1. Field of the Invention
The present invention relates generally to a so-called auto-focusing system, and particularly to a method and device for detecting focusing conditions of an image incident upon an image sensor, such as a CCD line sensor.
2. Prior Art Statement
In the auto-focusing system wherein an image sensor, such as a CCD line sensor, is used, it has been proposed to put out a time series output signal in response to the quantities of light corresponding to respective picture elements and sensed by the image sensor, the time series output signal being utilized as a contrast signal, and to find out the focusing position at which the contrast signal takes the maximum value. For example, U.S. Pat. Nos. 4,560,864 and 4,470,676 disclose systems wherein the absolute values of high frequency wave components of the time series output signal are integrated to be utilized as the contrast signals.
However, the effective output signals corresponding to effective picture elements contributing for the detection of real image informations are not fed throughout the whole range from the starting time to the terminating time of the reading-out period, but are fed only in a certain time duration within the reading-out period. For instance, referring to FIG. 3, an output signal B fed for a given reading-out period T.sub.0 between the transfer pulses A and A includes an effective output signal B.sub.1 which is fed within a limited time duration T.sub.1 and ineffective output signals B.sub.2 and B.sub.3 which are fed for time durations out of said limited time duration T.sub.1. The electric potentials of the ineffective output signals B.sub.2 and B.sub.3 are substantially zero, whereas the effective output signal B.sub.1 has a positive electric potential. Thus, the output signal B is shifted stepwisely at the point between the output signals B.sub.2 and B.sub.1 and also at the point between the output signals B.sub.1 and B.sub.3. As a result, the high frequency wave components C of the output signal B include high false signals C.sub.1 and C.sub.2 the generated due to the stepwise shift of the output signal B. The result of integration of the absolute values of the high frequency wave components C, according to the prior art technology, involves errors due to the integration of the false signals C.sub.1 and C.sub.2.
On the other hand, the practical output signal contains various noise signals. Such noise signals include, for example, fixed pattern noises which cause unevenness in brightness of a specific picture element, internal noise signals generated internally of the image sensor, and preamplifier noise signals. The internal noise signals and the preamplifier noise signals are random noise signals, and thus such noise signals incidentally and continually be always contained in the output signal. In contrast thereto, the fixed pattern noise signals are contained in the output signal for a specific picture element generated from a given image sensor.
Since the contrast signal obtained by the prior art technology contains various noise signals as described above, the contrast signal is affected seriously by the random noise signals when the reading-out period is set for a long time, and also affected by the fixed pattern noise signals if the time or range for integration is varied. For this reason, accurate focus control cannot be achieved by the prior art technology.